Exchange 2016 – Delegates corruption

When a user adds another user to their mailbox as a delegate, and chooses to have meeting requests sent to their delegates, a hidden inbox rule is created in their mailbox to perform the forwarding of those meeting requests. This can also apply to delegates room and equipment mailboxes.

When the delegate is later removed from the list, they might continue to receive the meeting requests. If the former delegate’s mailbox has been removed from the organization, then people who send meeting requests to the mailbox for which they were previously a delegate might receive a non-delivery report.

The root cause of both issues is a stale, or corrupt, delegate entry still being included in the hidden inbox rule. There are two solutions you can try to fix this:

Remove all delegates from the mailbox, and then re-add them.

Use MFCMAPI to remove the hidden inbox rule (which also removes all delegates, requiring you to re-add them).

To use MFCMAPI to remove the hidden inbox rule as following:

Export all rules from you Outlook so that you may import them back after deleting all rules via MFCMAPI.